Answer
$f= -10.46, f= 14.46$
Work Step by Step
Use either the square root method, the quadratic formula or factoring. $$
\begin{aligned}
120 & =-28 f+7 f^2-939 \\
120+939 & =7 f^2-28 f \\
1059 & =7 f^2-28 f\\
f^2-4 f & =\frac{1059}{7} \\
f^2-4 f+2^2 & =\frac{1059}{7}+2^2 \\
(f-2)^2 & =\frac{1059+4\cdot 7}{7} \\
(f-2)^2 & =\frac{1087}{7} \\
f-2 & = \pm \sqrt{\frac{1087}{7}} \\
f & =2 \pm 12.46.
\end{aligned}
$$ This gives:
$$
\begin{aligned}
f & =2-12.46 \\
& =-10.46 \\
f & =2+12.46 \\
& =14.46
\end{aligned}
$$
